The present invention relates to a system for detecting a fault of a power transmission and distribution apparatus such as a gas-insulated switchgear apparatus, or more in particular to an energization fault detection system for disovering at an early time an energization fault including a contact failure due to a contact abnormality in a conductor connection thereby to prevent a serious accident such as grounding.
With the advance of information society, demand for higher power quality is mounting and power transmission and distribution apparatuses such as gas-insulated switchgear apparatus are required to be more and more reliable. In order to meet this demand, a preventive maintenance technique is important for early detection of a fault, if any, and preventing a serious accident such as the grounding. One of the faults to be detected is an energization fault. This fault occurs when a large current flows in a part where a contact failure is caused by (i) the lack of the contacting force due to the insufficient tightening of bolts in the process of assembling a conductor connection, (ii) the shortage of the contact area due to an incomplete throwin attributable to a malfunction of an operating unit of a switching section, or (iii) the damage of the contact area of the contactor of a circuit due to turning off an excessive current. Conventional methods of detecting an energization fault by detecting a temperature increase due to generation of heat at a faulty part are disclosed in JP-A-56-68131 (UT) and JP-A-55-154428. The system disclosed in the former reference, which is equipped with a temperature sensor on the outer wall of an equipment tank for securing reliability, is easily affected by the elements and is low in sensitivity as the sensor thereof is at some distance from a component part subjected to an energization fault, thereby posing the disadvantage of insufficient fault detection accuracy. The latter reference relates to a system with a sensor mounted on a high-potential conductor in order to improve the detection accuracy and gives rise to the problem of low reliability as the sensor maintenance is difficult. If an insulating gas such as SF.sub.6 gas sealed in the device housing is exposed to a faulty part increased in temperature, it is decomposed and generates a decomposition gas. A method of detecting such a decomposition gas is disclosed in JP-A-55-41165. In addition to the problem of the sensitivity for detecting a decomposition gas of several ppm and how to secure the reliability against the deterioration by corrosion of the sensor due to the insulating gas, this method has the disadvantage that the overwhelming amount of the decomposition gas generated at the time of current cut-off in a gas section containing a circuit breaker and a disconnector renders this system essentially impracticable. As to other gas sections, the systems available on the market is lacking in sufficient detection sensitivity. A method of detecting an electromagnetic wave or an electrical pulse is disclosed in JP-A-55-117421. An electrical signal detectable by this method is generated in the last stage of progress of a fault when a minute spark discharged begins to occur between electrodes, and therefore even if the fault is detected, it is only several to several tens of seconds before the grounding occurs. Within this short length of time, it would be impossible to prevent the grounding.
Another phenomenon to be detected by a preventive maintenance technique is an insulation failure. One such method based on detection of partial discharge by vibration, hereinafter oscillation, measurement is described in IEEE Trans. on Power Apparatus and Systems Vol. PAS-100, No. 6 (1981) pp. 2733 to pp. 2739. The oscillation referred to in this article is in the frequency range of approximately 10 kHz and less than 10 mG in acceleration generated by partial discharge. This system for detecting such a minute signal of a comparatively high frequency involves an oscillation different in both source and frequency region from the oscillation intended to be detected by the present invention.
The oscillation generated at the time of an energization fault is described in the Lecture No. 1147 before the 1988 National Conference of the Institute of Electrical Engineers of Japan. However, the lecture fails to refer to any method of detection of such an oscillation, or in particular to how to improve the accuracy of detection.